Large surface area per unit weight materials such as activated carbon are used as electrodes in a variety of applications, such as electrochemical double layer capacitors (EDLC), otherwise known as ultracapacitors, and lithium ion batteries. The carbon electrode surface is where charge exchange occurs with the electrolyte. In the case of an EDLC, charge separation occurs, not between the capacitor plates as such, but across the electrochemical double layer—typically on the order of nanometers thick. The very large surface area of an activated carbon electrode allows for very large capacitances and very high stored energy densities in EDLCs.
Activated carbon can simply be a powder-compact made up of extremely small and very “rough” particles; in bulk the particles fond a low-density compact of particles with holes between them that resembles a sponge. The activated particles can resemble a solid core surrounded by a shell with cracks. Overall the openings on the surface of particles, as well as those among them, lead to pores that are classified as micro (<2 nm), meso (2-50 nm) and macro (>50 nm) pores. See FIG. 2 for an example of an activated carbon particle 200, with a solid core 210 and “cracked” shell 220; the boundary, which is not necessarily as well defined as shown, between the core and shell is indicated by dashed line 230. The surface area of such activated carbon materials, measured by a method based on BET theory, ranges between 1000 and 2500 m2/g. See S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, vol. 60, pg. 309 for a discussion of BET theory.
Activated carbons are made from phenol resin, natural materials (coconut shells, wood), coal or oil pitch. Typically “carbonization” of such carbon-precursors followed by “activation” of the carbonized product are two distinct process steps. The processes currently used are time consuming, expensive, and result in low yield. The cost of EDLC grade activated carbon is in the range of $35-$100/kg, whereas the coal tar pitch or phenol resins costs less than $1 per kg.
In preparing an ultracapacitor, the EDLC-grade activated carbon is mixed with: typically 5 to 10 weight percent of binder materials, such as polyvinyledene, polytetrafluoroethylene, and/or Teflon®; an electrical conductivity enhancer, such as carbon black; and other additives, as required. This mixture may be ground, and/or ball-milled followed by dispersion in N-methylpyrrolidone (or similar) to form a slurry, or the mixture may be mixed with solvents such as isopropyl alcohol to form a paste, dough, or solution of desired viscosity (pliability). Often such mixing is done in an ultrasonic mixer. The slurry material is then applied to a metal foil, called a current collector, (typically aluminum or copper) on continuous winding machines to form electrodes. To ensure appropriate bonding between the slurry and the metal foil, some pressure is applied by passing the electrode between rollers—this process step is often referred to as “calendaring.” Lastly, the solvent is evaporated, the binder is removed, and the electrode is dried, generally at a high temperature.
Even though the electrodes are processed from nano-scale, micro-pore activated carbon particles that have the potential to provide large surface areas per unit weight or volume, they are often processed using basic powder processing methodologies, as described above. This processing leads to a significant reduction in the useful surface area of the carbon material, yielding much lower than theoretically possible capacitances. For example, application of roller pressure in the calendaring step alone leads to a reduction in porosity of more than 50% and reduces the BET surface area down to 500 to 1000 m2/g.
In conclusion, there is a need for improved processing techniques that provide activated carbon electrodes with surface areas closer to the theoretically calculated values. Such electrodes would provide a significant improvement in the stored energy density in EDLCs. Furthermore, there is a need for lower cost and higher yield processes for forming activated carbon and incorporating said activated carbon in an electrode. This would provide a significant cost reduction in EDLCs.